Alterations of low density lipoprotein (LDL) cholesterol and apolipoprotein (apo) B have been implicated in atherosclerosis. Hypobetalipoproteinemia is an autosomal codominant disorder characterized by an LDL level less than 50% of normal and low apoB levels. The specific aims of this project are: 1) to determine the mechanism for the low LDL and elevated high density lipoprotein (HDL) levels in the apoB-67 kindred; 2) screen a Founder population with a high incidence of apo B-67 for other apoB mutations; and 3) screen the Framingham population to determine the prevalence of truncated apoB mutations. Mutations detected will be characterized and related to the presence or absence of coronary artery disease (CAD). The apoB-67 kindred was identified and characterized by the applicant who, in Stephen Young's laboratory, cloned and sequenced a portion of the apoB gene and found a one base deletion at cDNA 9327 that causes a premature stop codon and a truncated apoB-67, in addition to apoB-48 and B-100 in plasma. Affected members appear to remain thin and free of heart disease on a high caloric, high fat and cholesterol diet. The metabolism of VLDL B-48, B-67 and B-100 and apoA-I will be evaluated by labeling apoB by a constant intravenous infusion of D3-leucine. The rate of incorporation of D3-leucine into VLDL, LDL and HDL provides a direct measurement of their synthetic rates from which catabolic rates will be calculated. New mutations detected will be characterized at the molecular level by amplification, cloning, sequencing and kinetics. Those members over age 60 with truncated apoB mutations will be evaluated with echocardiography and exercise stress testing for coronary artery disease. The hypotheses to be tested are that 1) the apoB mutation is related to decreased production of apoB and decreased catabolism of HDL and 2) low LDL levels are related to the absence of coronary artery disease. Information gained from this project may prove that 1) hypobetalipoproteinemia is associated with longevity due to prevention of CAD secondary to low LDL levels; 2) there is a reciprocal relationship between LDL and HDL metabolism; and 3) a single mutant apoB allele may protect against deleterious effects of a saturated fat and cholesterol- rich diet. The applicant hopes to acquire experience in the sponsor's laboratory which is active in lipoprotein metabolism and molecular biology. This preceptorship would provide the essential knowledge and skills needed to become an independent investigator in lipoprotein metabolism and molecular cardiology.